As cellular communications have developed, there have been an increasing number of users for each cell. This increased number of users has begun to strain the quality of the signals in those cells, as the increased number of users creates interference (sometimes referred to as co-channel interference) between the users.
Co-channel interference can be limited through the use of beamforming technology. Beamforming employs an antenna array with multiple antenna elements at a base transceiver station (BTS) wherein the signals received from the multiple antenna elements are dynamically weighted and combined, to enhance signals originating from desired mobile subscribers, while suppressing signals from unwanted mobile subscribers. An antenna array with such so-called adaptive beamforming capability is sometimes referred to as a smart antenna.
In a Time Division Multiple Access (TDMA) system such as the Global System for Mobile Communications (GSM), a known training sequence is inserted in each time slot to facilitate channel estimation, which may be used to assist in signal processing. It is also known that this training sequence may also be used to allow the base transceiver station to calculate the uplink beamforming weights, in order to optimize the signal strength for the desired users.
The majority of existing base transceiver stations do not have beamforming capability and operators are often hesitant to invest in an entirely new base transceiver station with enhanced capability. As such, beamforming systems are typically implemented on a cell-by-cell basis, preferably by introducing the advanced beamforming capability in a separate or appliqué structure that can be inserted in-line with the signal path without modification of the existing components.
When introducing an appliqué system to a base transceiver station, the amount of signal delay permitted to the appliqué system before entering the base transceiver station is constrained, which prevents store-and-forward type of processing where the received signals are temporarily stored until the training sequence has been received. Rather, conventional appliqué systems attempt to perform their processing in real-time. Thus, the beamforming weights are to be calculated, applied to the signals and the signals are combined and forwarded to the base transceiver station, all within the permissible time constraint.
If the co-channel interference is stationary in time, the weights obtained in the previous frame could conceivably be applied to the current frame of signals with little performance degradation, in order to satisfy this time constraint.
However, network administrators have increasingly adopted Slow Frequency Hopping (SFH) techniques to improve average channel quality for users by minimizing the likelihood that a user languishes in a channel with poor signal quality. Rather, the user is constantly moved to a different frequency. In a wireless system where slow frequency hopping (SFH) is used, the desired mobile may experience co-channel interference from different mobile subscribers in consecutive time frames due to the time-varying nature of the transmit frequency of each mobile subscriber. In this case, it is no longer appropriate to apply the beamforming weights from a previous frame to the data of the current frame, as the frequencies and thus potential interferers may well have changed in consecutive frames.